1. Field of the Invention
The present invention relates to a method of making a hollow, interiorly coated glass body from a glass tube made of a low melting glass material and acting as semifinished product or intermediate product.
The invention also relates to a glass tube made from low melting glass material and acting as a semifinished product for forming a hollow glass body with an interior coating having a high chemical resistance or inertness.
2. Prior Art
Low melting glass materials, such as borosilicate glasses or calcium, sodium glasses, corrode in a known manner on contact with water or other liquids. Particularly water withdraws sodium ions from glass.
Thus it is necessary for numerous applications to increase the chemical resistance of the glass bodies, which are formed from this type of low melting glass, especially hollow glass bodies formed from glass tubes.
Hollow glass bodies, which require an increased chemical resistance for the interior surface, are, for example, those used
for chemical plant structures, PA1 for flow meters for chemically reactive media, PA1 for analytical purposes (e.g. burette tubes, titration cylinders, etc.), PA1 for reagent glasses for special purposes, PA1 for sheathing of measuring electrodes in reactive media, PA1 for illumination purposes, e.g. halogen lamps, PA1 for discharge lamps, PA1 for components used for biotechnology reactors, and PA1 as containers for medicinal purposes (e.g. ampoules, bottles, injector devices, cylindrical ampoules, etc.). PA1 coating the interior surface of the semifinished glass tube with an oxide material to form an interior coating having a coating thickness which is adapted to the subsequent shaping or working conditions required for making the glass body and the chemical resistance requirements of the glass body, and PA1 making the glass body from the interiorly coated semifinished glass tube. PA1 Methods for coating from the liquid phase (Sol Gel coating), for example, are described in H. Bach, D. Krause, "Thin Films on Glass", Springer Verlag, Berlin (1997). PA1 Methods are known for precipitation from supersaturated solutions. PA1 Sputtering methods, even when their use for pipe-like substrates is complicated, can be used, since sputtering process are direct processes. PA1 Advantageously CVD processes (CVD=chemical vapor deposition) can be used for making of the semifinished glass tube. The coating is produced at elevated temperatures (i.e. higher than room temperature) in so-called thermal CVD methods. These methods can be used directly during the manufacture of the glass tube after the known drawing process. For this purpose the coating gas is used as supporting air/blowing air. The coating gas decomposes in a predetermined temperature range in the glass tube and forms a coating on its interior tube surface. A suitable similar method can of course be employed which is independent of the manufacture of the glass tube however re-heating of the glass tube is then required. The subsequent heating can occur by different methods, e.g. direct heating, heating with a laser and so forth. It is also possible to reduce the coating temperature when light radiation is used for activation/production of the active coating conditions. PA1 Advantageously the deposition of the oxide coating material can occur from the gas phase, from the coating gas, by means of a vacuum-assisted plasma CVD method, the so-called PECVD processes (plasma enhanced chemical vapor deposition). The PECVD process is described in various references. Diverse embodiments are used with different energy input in the low frequency range (e.g. 40 kHz), in the middle frequency range (e.g. 13.56 MHz) up to the microwave range (2.45 GHz and above). Examples are found in G. Janzen, "Plasma-technik(Plasma Engineering)", Hutig-Verlag, Heidelbery, 1992. PA1 In a preferred embodiment which is especially advantageous a modified PECVD method, the so-called PICVD process (plasma-impulse-CVD process) is used, which provides a high uniformity for large-scale coated substrates. The PICVD technology is known in the patent literature from German Patent Document DE 40 08 405 C1 and from U.S. Pat. No. 5,154,943 and for example used for producing barrier layers on plastic containers (German Patent Document DE 44 38 359 A1). This technology uses pulsed plasmas for deposition of coatings from the respective coating gases.
The latter mentioned applications are of special significance.
It is indeed known to make glass tubes from silica glass (quartz glass, SiO.sub.2 glass) as a semifinished product for forming hollow glass bodies, which have a very high chemical resistance. Those glass tubes are however very expensive because of the high melting point of the SiO.sub.2 glass. Furthermore they can only be made with limited optical quality and are less suitable for mass production. These tubes may be formed with only very special apparatus since, on the one hand, their forming temperatures are very high and, on the other hand, the temperature interval in which their formation is possible is very small.
Semifinished glass tubes made from silica glass thus may not be of sufficient quality and are uneconomical for mass applications.
Predominantly low melting glasses, e.g. borosilicate glasses or calcium-sodium glasses, are used for large-scale glass products. These may advantageously be formed as tubes economically.
For example these glasses include the following: Duran.RTM.-borosilicate glass (Schott Glas), Fiolax.RTM.klar(Schott glass, Fiolax.RTM.braun(Schott Glas) and Kimble N 51 A (Fa. Kimble).
The compositions of these glasses made in the form of glass tubing are tabulated in the following Table I.
TABLE I GLASS COMPOSITIONS IN % by WEIGHT* GLASS SiO.sub.2 B.sub.2 O.sub.3 Al.sub.2 O.sub.3 Na.sub.2 O K.sub.2 O MgO CaO BaO 1 69 1.0 4 12.5 3.5 2.5 5 2 2 69 1.0 4 12.5 3.5 2.5 5 2 3 69 1.0 4 12.5 3.5 2.5 5 2 4 70 1.0 4 12.5 3.5 2.5 5 2 5 69 1.0 4 12.5 3.5 2.5 5 2 6 69 1.0 4 12.5 3.5 2.5 5 2 7 75 11 5 7 1.5 0.5 8 75 11 5 7 1.5 0.5 9 80 13 2.5 3.5 0.5 10 70.8 8 5.5 7 1.5 1 2 11 70.8 8 5.5 7 1.5 0.5 2 12 72.8 11 7 7 1 1 13 73.3 10 6 6 3 0.5 14 74.3 10 6 8 1 *balance to 100% consists of other elements (for No. 10 and No. 11 Fe.sub.2 O.sub.3 and TiO.sub.2 which together are 3.5%)
It is known to increase the chemical resistance of these glass tubes made from low melting glass by a method in which the glass surface is chemically leached out. A sutable reactive gas (SO.sub.2, (NH.sub.4).sub.2 SO.sub.4 or HCl) is conducted through the still warm glass tube, which leads to a surface reaction and a reduction in the alkali content at the surface.
This type of dealkalizing process is, e.g., described in H. A. Schaeffer, et al, Glastechn. Ber. 54, Nr. 8. pp. 247 to 256. The disadvantage of this process is that predominantly toxic gasses are used, whereby the glass surface can contain traces of these reactive reaction gases after this chemical treatment and the glass surface structure is damaged which leads to an increased surface area and to an increase in reactive sites on the surface. Furthermore the use of these reactive gases is undesirable from an environmental standpoint and due to worker safety consideration. With many of the suggested gases corrosive by-products arise, which react strongly with metal apparatus parts. Furthermore particles can be released from the porous damaged surfaces during shaping or forming of this type of leached out glass tube. Also a washing process for removal of reaction products is necessary prior to use of the leached out glass tube. This washing process necessitates a drying and disposal of reaction products, i.e. the costs increase for making the semifinished glass tubes.
An additional process for dealkalizing low melting glass by fluorination by means of fluoro-acids, which has the same main disadvantages as the above-described process, is described in U.S. Pat. No. 3,314,772.
In order to avoid the disadvantages of dealkalizing process it is also known to provide a tubular glass container from low melting glass material, which operates as a packaging device for pharmaceutical materials, having a silicon dioxide (SiO.sub.2) layer on its interior surface, which has the same inertness as a quartz glass surface (M. Walther, "Packaging of sensitive perenteral drugs in glass containers with a quartz-like surface", in Pharmaceutical Technology Europe, May, 1996, Vol. 8, Nr. 5, pp. 22 to 27.
The coating of the interior surface of the formed glass body occurs by chemical deposition of an oxide coating from the gas phase, especially by means of a vacuum-assisted plasma CVD process (PECVD=plasma enchanced chemical vapor deposition), in particular by means of a pulsed plasma process (PICVD=plasma impulse chemical vapor deposition).
This PECVD or PICVD method for coating of an interior of a hollow body, especially made from plastic, is known from German Patent Documents DE 196 29 877 and DE-Z "Multilayer Barrier Coating System produced by Plasma-impulse Chemical Vapor Deposition(PRCVD)" by M. Walther, M. Hemming, M. Spallek, in "Surface and Coatings Technology" 80, pp. 200 to 205 (1966).
In the known case (DE 296 09 958 U1) the finished containers, i.e. the glass bodies themselves, are interiorly coated. Because of that each glass container, must be subjected to an expensive coating process, adapted to its form.